Shaping Thermal Transport and Temperature Distribution via Anisotropic Carbon Fiber Reinforced Composites

With the ongoing electrification of vehicles, thermal management is on everyone’s lips. To prevent overheating in electronic systems, new design strategies for thermal dissipation are needed. Thermally anisotropic materials enable targeted directional heat transport due to their anisotropic thermal conduction. Laminates made of unidirectionally aligned carbon fibers in a polymer matrix can be tailored regarding their in-plane anisotropy. Exposing the laminates to a temperature gradient reveals that the thermal transport is determined by their anisotropic properties. The corresponding heat flow can be visualized by IR thermography. The combination of anisotropic laminate discs into composite materials, similar to building with toy bricks, enables precise control of heat transport in the macroscopic composite materials. Thus, we achieve control of heat flow at the level of the individual components. In addition, we show that the orientation of anisotropy relative to the temperature gradient is crucial to guide the heat flow selectively. We found that the ratio of thermal anisotropy, the amount and arrangement of anisotropic components, and their positioning in the composite strongly influence heat transport. By combining all these factors, we are able to locally control the heat flow in composites by creating materials to either dissipate heat or block heat transport. The proposed concept can be extended to different shapes of building blocks in two or three dimensions.

Instead of discs, one may use blocks, hexagons, or other shapes suited for specific needs.Figure S5 gives an overview of several composite structures for three types of building blocks.
Discs offer the most variability in their arrangement, they can either be distributed in a simple cubic lattice or a hexagonal lattice.Additionally, randomly packed structures are most easily achieved using discs (or spheres in 3D, respectively).Starting with a simple cubic arrangement of discs, one easily obtains a square lattice made of squares by increasing the sphere radius until they overlap completely.In the same way, the hexagons can be created on the base of discs arranged in a hexagonal lattice.The temperature distributions, however, distinguish a lot from each other.In the isotropic case (top row of Figure 5, the influence of non-conducting pores in a material is visible.For the squares and hexagons, the temperature distribution is nearly perfectly homogeneously distributed between the hot and cold edges.In the case of the discs in a cubic simple arrangement, the heat has to flow through the narrow contacts between the particles, resulting in a slight change in the temperature distribution around the contact points.The heat transport to the sides is in all cases negligible, as the temperature gradient imposes the direction of heat flow from top to bottom.A similar effect is found in the disc structure in a hexagonal lattice.The introduction of a parallel line of anisotropic building blocks leads to minor effects on the respective temperature distributions.Conversely, the implementation of only two anisotropic building blocks in the left corner (orientation 45 • influences the temperature distributions drastically.
As expected, for the squares and hexagons the temperature distributions display the orientation of the anisotropy directly.In the case of the discs arranged in a hexagonal lattice, the anisotropy of the middle disc is also directly visible.The temperature distribution strongly resembles that of the hexagons, with small additional effects imposed by the non-conducting spaces between the discs.Similarly, the temperature distribution of the discs arranged in a square lattice resembles the one of the squares.However, the amount and orientation of anisotropic discs can not be seen at a direct glance.Similar observations are found in the case of the structures with purely anisotropic components.
To sum up, the shape and arrangement of the building blocks can be chosen freely.Each of them results in a slightly different temperature distribution that is mainly determined by the position and broadness of the contact areas between the single building blocks.

Heat flow in a single square: angle dependency of anisotropy
We observed a very interesting artifact when analyzing the different types of building blocks in detail at the component level.Beginning with a simple square with thermal anisotropy, the following temperature distributions are obtained, depending on the respective orientation of the preferred conduction axis (see Figure 4.Note that the temperature gradient here is imposed from right to left, for reasons of simplicity regarding the notation of the angles.
The results for 0 • as well as for 90 • are homogeneous temperature distributions, as expected.
However, for angles between 0 • and 45 • unexpected artifacts occur.Naturally, the heat is conducted along the axis of preferred heat conduction, but as long as the angles are such that the heat is led to the thermally insulated sides of the square, the main heat flow occurs on the left side, being the shortest path using the unfavorable bad conductivity axis.A detailed look at the boundaries of the temperature gradient reveals that there are regions close nearby with enhanced temperature.We found the highest, and respectively lowest temperature region for 45 • .Here, both maxima exceeded the applied temperature conditions for the formation of the steady-state temperature gradient.The same effect can be found for rectangles, however at different angles, depending on the aspect ratio of the rectangle.For angles larger than the diagonal through the block, the heat transport can follow unhindered the direction led by the preferred conduction axis.Consequently, the effect of heat accumulation is not observed anymore.
Presumably, this effect only shows up for completely thermally isolated systems with a shape that blocks the heat transfer from top to bottom completely.We did not observe heat Supplementary Figures Figure S1: a)Thermograms of single anisotropic discs surrounded by six isotropic laminate discs.The temperature distribution is changed locally by the orientation of the preferred conduction axis of the anisotropic disc.b)Temperature distribution of a line of neighbored discs with anisotropy orthogonal to the temperature gradient.

Figure S3 :
Figure S2: Thermograms illustrating the strong effect on the temperature distribution of blockers (discs with the preferred thermal conduction orthogonal to the temperature gradient.The use of blockers leads to strong local temperature gradients in single building blocks.

Figure S4 :
Figure S4: Temperature distributions of a single anisotropic square for different angles of the preferred thermal conduction axis.